The invention relates to a method and apparatus for covering a metal roll core with a polymeric material such as a high performance thermoplastic material. However, the present invention is not limited to the use of high performance thermoplastic materials and contemplates the use of other extrudable elastomers such as rubbers or processable urethanes, and high viscosity thermoset resins such as polyurethanes and epoxies in general. These covered rolls are utilized in many applications including an application known as calendering. Calendering is the act of pressing a material, e.g., cloth, rubber or paper, between rollers or plates in order to smooth or glaze or to thin into sheets. The covered rolls that are discussed in this application are also known as calender rolls, soft-nip calender rolls and supercalender rolls and are often utilized in industrial environments such as paper mills. However, the invention described herein is not limited to covered rolls known by these names or utilized in these environments.
In a typical paper mill, large numbers of rolls are used not only for transporting the web sheet which becomes paper, but also for calendering or processing the web sheet into a specific grade of paper. The finished paper product must possess certain quality characteristics such as a high degree of caliper uniformity, bulk uniformity, smoothness, gloss and printability. In order to achieve these quality characteristics, it is necessary that the calender roll be precisely manufactured utilizing materials that can withstand severe conditions during paper processing.
For example, when used for transporting the web sheet, it is essential that these covered rolls provide traction to enable the transport ofthe paper during processing. Additionally, these rolls must be wear and corrosion resistant. During use for calendering, these covered rolls are subjected to high dynamic stress, heat, speed, abrasion and impact and therefore must be fabricated to withstand these elements. In order to function properly for these specific uses, the covered rolls must have an appropriate surface hardness based upon the intended application for the covered roll and also have a high thermal resistance to withstand high temperatures and pressures in the environments in which they are employed. Regardless of their application, these covered rolls are precision elements of the systems in which they are utilized and therefore must be precisely manufactured to achieve balance, specific size and shape specifications, surface characteristics and tight tolerances. The covered rolls have similar transporting and calendering functions in the textile industry as well as in facilities where magnetic tape is manufactured.
Conventional prior art calender rolls comprise a metal cylinder to which either a cotton-filler or a thermoset composite layer (or layers) is added to preclude metal-to-web-to-metal contact at the nip between the calender rolls during the calendering operation. Though cotton-filled roll covers have been used for a long time there are several drawbacks associated with their use such as the need for frequent regrinding. Moreover, cotton filler material is not sufficiently tough to withstand the high stress and high temperatures associated with demanding applications such as in modern paper fabrication. Paper mills must frequently regrind and replace cotton-filled roll covers, even when they perform well. This results in significant production down-time and higher costs associated with keeping replacement rolls in inventory.
Over the last two or three decades, synthetic composite roll covers have been developed to resolve many of the problems associated with cotton-filled roll covers. Most of these synthetic composite roll covers use some form of thermoset resin such as epoxy, rubber or polyurethane among others, as. a base material which is combined with some form of reinforcement material to improve strength.
As an example, a synthetic composite roll cover is formed of a single layer of a reinforcement fiber mat that is impregnated with a thermoset epoxy which is then cured. The surface of the cured single layer is then machined to a smooth finish in accordance with customer specifications.
Alternatively, rather than machining the cured single layer to a smooth finish, an additional layer of reinforcement fiber mat may be added over the cured single layer, the additional layer being impregnated with an epoxy which is then cured to form a top layer. The surface of the cured top layer, which provides the outer surface of the roll is then machined to a smooth finish in accordance with customer specifications. The single layer, which forms an under layer, provides a transitional element between the metal core and the top layer to assist in establishing an effective bond and stress distribution between the two layers of the covered roll. Alternatively, additional layers could be added.
In practice, a layer of the synthetic composite cover is added to the roll core by unspooling a strip of dry reinforcement fiber mat, several inches in width, from a reel and conveying the unspooled strip through an epoxy bath. The roll core is oriented horizontally and rotated to wind the epoxy impregnated strip onto the roll core in a back and forth fashion to form a helix. The epoxy is then allowed to cure to form a layer which is then machined to a smooth finish.
The use of these synthetic composite roll covers has increased dramatically in the last ten years because of their superior performance characteristics over conventional cotton-filled roll covers. The acceptance and usage of synthetic composite roll covers in the paper industry has resulted in the beginning of the demise of the cotton-filled roll cover. Notwithstanding their superiority over cotton-filled roll covers, synthetic composites such as thermosetting epoxies also suffer from several drawbacks. For example, to formulate a synthetic composite roll cover having certain desirable properties such as high toughness, high temperature capability (glass transition temperature (T.sub.g), it is usually necessary to employ a higher concentration of reinforcement fibers. Increasing the concentration of reinforcement fibers in this manner can result in the emergence of other less desirable properties such as unacceptable surface finish, easier delamination, greater brittleness, and poor bonding between the cover and the metal outer surface of the roll core. Roll manufacturers struggle to optimize these conflicting properties to achieve a superior roll cover. Failure and inconsistent performance of the synthetic composite roll covers in the field have been and continue to be a common problem. Even with recent advances in resin chemistry, synthetic composite roll covers are best performing at operating conditions wherein the maximum operating temperature does not considerably exceed 250.degree. F., the maximum nip pressure does not considerably exceed 10,000 p. s.i., and wherein the surface roughness of the cover is considerably less than 10 Ra micro-inches.
The method and apparatus of the present invention enables the fabrication of rolls covered with synthetic composite materials such as those discussed above. The method and apparatus ofthe present invention also enables the fabrication of rolls covered with high performance or engineered thermoplastic materials. Some thermoplastic materials have a number of highly desirable properties making them superior to synthetic composite materials being utilized today in the fabrication of covered roll cores. This includes a higher glass transition temperature, a more suitable Young's modulus for many applications, a higher tensile strength, greater smoothness, a higher impact strength, more uniform surface finish and more homogenous physical and thermal properties. Thus, a roll core covered with a high performance thermoplastic material will achieve superior performance characteristics than one covered with a synthetic composite material such as thermosetting epoxy resin.